It is an objective of the United States to reduce oil imports and where appropriate to increase the use of coal, resource abundantly available in the United States. Increasing the use of coal can be enhanced if methods which produce coal with a relatively low pyrite and/or mineral content be available, because coal low in pyrite or in mineral content can be used as replacement fuel for petroleum-derived fuels. For example, coal-water slurry can replace heavy residual oil in existing boilers with a minimum derating only if the total solid content of the slurry will be comparable to that of residual oil and the volatility of these solids will not exceed that of the solids in heavy oils. Since numerous boilers, many of which are very new, have been designed to burn heavy oil, finding a coal-based replacement-fuel which will allow the use of these boilers for a longer period of time is extremely desirable.
Besides coal-water slurry or turbine applications, there is a tremendous economic incentive to produce low pyrite and low ash (or low minerals) coal for other applications because such coal can reduce the overall cost of power generation in various applications and the quantity of sulfur oxides emitted upon its combustion.
Past developments in coal preparation processes along with advancement in mining technology have made it possible to produce what is referred to as low mineral content coal. Such coal typically has an ash content of 5 to 10 percent by weight at best with BTU recovery on the order of 60 to 80 percent. These results are not satisfactory because a large fraction of the coal is rejected, which hurts economically and increases the environmental problems associated with disposal of the refuse. Moreover, high ash coals cannot be used at all in many applications including coal-water slurry burners, turbines, and novel motors which use solid fuels. It should be noted that the operation of a coal slurry or a turbine burner is extremely sensitive to the coal ash content and composition. For example, lowering the ash content 1 percent may allow the use of the coal in some applications where it was not previously usable.
There is a real need for a coal demineralization process that is feasible, practical and cost-effective that will reduce the mineral content of coal to below 3 percent by weight, and the pyrite content to below 0.3% while providing a BTU recovery level of at least 70 to 75 percent.
Technologies capable of reducing the ash content of coal to below 3 percent exist today. However, they are expensive, capital-intensive and not fully proven. First, there is what is referred to as "solvent refining" of coal, and secondly, there is a base-fusion process in which the coal is treated by a melted alkali or by a very concentrated alkali solution at elevated temperatures and pressures. Typical costs associated with solvent refining of coal are $40 to $130 per ton with initial capital investment of the order of several hundred million dollars and up to three billion U.S. $/10000 t/day preparation plant. Coal demineralization by alkali fusion is even less attractive economically and is in a more primitive state of development.
Therefore, there is a significant and real need for a novel coal cleaning process which reduces the ash level to below 3 percent by weight; such process should have a high BTU recovery; should require a relatively small capital investment, and has a very low operating cost.